1. Field of the Invention
The present invention relates to a device and a method for optical recording and a signal processing circuit, and more particularly, to a device and a method for optical recording and a signal processing circuit in which a recording condition can be optimized depending on the compatibility between a drive and a medium.
2. Description of the Related Art
For recording onto optical information recording media such as CD-R or DVD-R (hereinafter referred to as “media” or a “medium”), the compatibility between a medium to be recorded on and a device to be used for recording (hereinafter referred to as a “drive”) depends on each combination of them. This may be caused by the medium side factor wherein an optimum recording condition is varied by the difference in a recording material comprising the medium or the film formation variation during production, or by the drive side factor wherein the optimum recording condition is varied by the difference in a pickup device and/or a semiconductor laser comprising the drive or assembly variation during production. In fact, a suitable recording condition exists for each combination of a medium and a drive due to the combination of those factors.
Accordingly, in a conventional method, ID information of a medium type identifiable by a drive is stored in the medium as well as a recording condition prepared for each medium type is stored in the drive. When recording is actually conducted, the ID information stored in the medium is read from the medium loaded to the drive, and a recording condition related to the ID information (hereinafter referred to as a “write strategy”) is used.
However, in the conventional method, suitable recording conditions can be selected for pre-verified known media, but prepared recording conditions may not be adequate to accommodate unknown media that are not pre-verified. Further, depending on the changes in recording environments such as a recording speed, disturbance, and change over time, the prepared recording conditions may not be adequate even for known media.
A method contemplated to accommodate such an unknown medium is described in Japanese Unexamined Patent Publication No. 2003-30837 and Japanese Unexamined Patent Publication No. 2004-110995.
As described in the paragraph [0020] of Japanese Unexamined Patent Publication No. 2003-30837 “ . . . a phase error relative to a channel clock is detected for every recording pattern. A recording compensation parameter adjustment section 12 optimizes an emission waveform rule on the basis of the detection result at the phase error detection section 11,” a method for detecting a phase error by comparing with a channel clock and for correcting the phase error is disclosed.
Also, the paragraph [0024] of this document describes that “Next, a test pattern for determining an emission waveform rule is recorded. The area wherein the test pattern is recorded is reproduced, and the relationship between a prepared emission waveform rule and a phase error amount is examined. In other words, the phase error amount in the combination of the length of each mark and the length of each space immediately before the mark is measured. An emission waveform rule wherein the phase error amount becomes zero is estimated from the measured phase error amount, whereby a desired emission waveform rule is determined . . . ”, disclosing a method for measuring a phase error amount for every combination of a mark and a space, and then estimating an emission waveform rule wherein the phase error amount becomes zero (see FIG. 16 and FIG. 20).
The method disclosed in Japanese Unexamined Patent Publication No. 2003-30837 is effective for optimizing a strategy because correction is made based on a phase error of a recording pattern.
Also, the paragraph [0045] of Japanese Unexamined Patent Publication No. 2004-110995 describes that “ . . . a top pulse corresponding to a 3T period and a non-multipulse corresponding to an 8T period are generated integrally (consecutively) . . . ” and the paragraph [0046] of the document describes that “ . . . the laser power for a write pulse is adjusted in two levels, and when the ratio between a laser power (a height value of the top pulse) Ph and a laser power (a height value of the non-multipulse) Pm is optimum, an optimum power can be obtained . . . ”, suggesting the effectiveness of optimizing the Ph/Pm ratio.
On the other hand, as a code identification method for a high density recording system using a blue laser, adoption of PRML (Partial Response and Maximum Likelihood) method has been considered. Since code identification is performed based on amplitude information of an RF signal obtained by reproducing a recording pattern in the PRML method, in order to provide high-quality recording, a recording condition should be configured using a metrics different from that used in a conventional slice method.
A method for configuring a recording condition based on amplitude information of an RF signal is described in the following patent documents:                Japanese Unexamined Patent Publication No. 2004-13978,        Japanese Unexamined Patent Publication No. 2004-280876,        Japanese Unexamined Patent Publication No. 2003-15129,        Japanese Unexamined Patent Publication No. 2004-63024,        Japanese Unexamined Patent Publication No. 2004-213759, and        Japanese Unexamined Patent Publication No. 2004-152473.The Patent Documents 2004-13978 and 2004-280876 disclose methods for determining a top pulse, an intermediate pulse, and a last pulse using an asymmetry as an indicator. The Patent Documents 2003-15129, 2004-63024, and 2004-213759 disclose a method for determining a condition of a recording pulse based on the difference between an ideal waveform on the assumption of PRML and a reproduction waveform obtained from actual recording. Japanese Unexamined Patent Publication No. 2004-152473 discloses a method for determining a start position of a top pulse using an asymmetry as an indicator as well as widths of a top pulse and an intermediate pulse using a jitter as an indicator.        
An example of performing code determination using a Viterbi decoder is disclosed in Japanese Unexamined Patent Publication No. 2002-197660. This document discloses a method for performing code identification of a reproduction signal as well as detecting amplitude and asymmetry of the reproduction signal by synchronizing an output from a Viterbi decoder to which a digitally-sampled reproduction signal is input with the digital reproduction signal of which timing is adjusted using a delay device.
However, because the method disclosed in the Japanese Unexamined Patent Publication No. 2003-30837 involves, as conventional methods, a fine adjustment of a prepared strategy stored in a drive, it is difficult to provide favorable recording quality for media to which prepared strategies are not applicable.
In the method disclosed in Japanese Unexamined Patent Publication No. 2004-110995, as described in the paragraph [0067] of the document, initial values of Ph and Pm are tentatively set based on values stored in a drive or a medium, and then the Ph/Pm ratio is calculated. Accordingly, as in the case of the Patent Document 1, it is difficult to provide favorable recording quality for media to which the tentatively set values are not applicable.
In the methods disclosed in the Patent Documents 2004-13978 and 2004-280876, because conditions of a top pulse and a last pulse are simultaneously determined using the same indicator, a shortest pulse in which an error is most likely to occur is not optimized. Thus, the methods are vulnerable to a disturbance and more likely to cause errors, and therefore cannot easily provide a high-quality recording system with a large recording margin.
In the methods disclosed in the Patent Documents 2003-15129, 2004-63024, and 2004-213759, because correction is made only for generally known pulse shapes, it is difficult to specify which part of a pulse shape is effective for which indicator. Thus, it is difficult to provide a high-quality recording system with a large recording margin just as in the methods disclosed in the Documents 2004-13978 and 2004-280876 described above.
The method disclosed in the Japanese Unexamined Patent Publication No. 2004-152473 also cannot easily provide a high-quality recording system with a large recording margin, similarly to the methods in the Documents 2004-13978 and 2004-280876, because the widths of a top pulse and an intermediate pulse are simultaneously determined using a jitter as an indicator although a start position of the top pulse is determined using an asymmetry as an indicator.
In the method disclosed in the Japanese Unexamined Patent Publication No. 2002-197660, reproduction signal quality is supposed to be high enough for Viterbi decoding. However, if the reproduction signal quality is low, there is a problem that an error occurs in code identification, resulting in a failure of detecting an amplitude level of a desired data code. Especially, in unknown media for which a recording strategy is not pre-stored in a memory of a recording device, an error in code identification is more likely to occur.
Further, to accommodate the increase of a channel bit rate with increasing the density of recording data, there are problems that when a low-speed A/D converter is used, obtaining an adequate number of samples for level detection becomes more difficult as the length of a signal becomes shorter, and when a high-speed A/D converter is used, it causes a significant increase in cost.
It is therefore an object of the present invention to provide a method to optimize recording conditions depending on the compatibility between a drive and a medium, which is especially effective for the case where a code identification method is used, such as PRML, in which codes are identified based on amplitude information of an RF signal.